1. Field of the Invention
The present invention relates to a battery state monitoring circuit that detects a voltage and an abnormality of a secondary battery including a nickel hydrogen battery and a lithium ion battery, and more particularly, to a battery state monitoring circuit capable of controlling charge and discharge to and from a battery pack containing a plurality of the secondary batteries that are connected in series.
2. Description of the Related Art
Nowadays, a secondary battery is used as power supply to a portable electronic device including a mobile phone, a notebook computer, and a portable game device. A lithium ion secondary battery is the mainstream of the secondary battery because of its light weight and its high energy density. The secondary battery has a risk of electrolyte leakage as a result of battery expansion after the battery has been charged excessively (overcharged). Besides, characteristics of the battery itself deteriorate after the battery has been discharged excessively (overdischarged). In view of the above, there is employed a protection circuit that protects the secondary battery from being overcharged and overdischarged. In particular, the lithium ion secondary battery is vulnerable to the overcharge and the overdischarge, and hence the protection circuit absolutely needs to be used for the lithium ion secondary battery.
Of portable electronic devices, there is used, for a notebook computer, a battery pack in which a plurality of batteries are connected in parallel and the parallel-connected batteries are connected in series. In a case of using the lithium ion secondary batteries in the battery pack, all of the batteries connected in series need to be monitored by protection circuits. Most of the battery packs used for the notebook computers contain three or four secondary batteries that are connected in series (three-series connection or four-series connection). Accordingly, a protection IC for four-series connection that includes protection circuits for respectively monitoring four batteries is commercially available.
Further, also as to electric power tools that are driven with high voltage, products using lithium ion secondary batteries have been introduced on the market. Studies are being performed on the use of an eight-series battery pack for the electric power tool.
In such an application as described above, when charge/discharge is controlled by means of one battery state monitoring circuit, the battery state monitoring circuit is required to have high withstand voltage characteristics. Therefore, there is employed a configuration in which a battery state monitoring circuit is provided to each secondary battery to thereby prevent the battery state monitoring circuit from being applied with a voltage exceeding its withstand voltage characteristics (see, for example, JP 2007-218680 A).
FIG. 5 illustrates a circuit example of a conventional battery device. The conventional battery device includes, for each secondary battery, a battery state monitoring circuit including input terminals DCHI and CHGI. Each of the battery state monitoring circuits transmits monitoring information, to thereby control charge/discharge of a large number of the secondary batteries that are connected in series.
In the conventional battery state monitoring circuits, however, the transmitted monitoring information and detected monitoring information are output to output terminals DCHO and CHGO via AND circuits 510 and 516, to be transmitted to a next battery state monitoring circuit or drivers 605 and 606. Therefore, there arises a problem that, when a voltage of the secondary battery becomes lower than a minimum operating voltage of the logic circuits, voltages of the output terminals cannot be secured, which makes impossible to inhibit charge.